Conventionally, light-emitting elements which use light emission from semiconductor rods have been developed (see Patent Literature 1). For example, a light-emitting element disclosed in Patent Literature 1 is a light-emitting element including semiconductor rods having p-n junctions and formed on a semiconductor substrate, with an electrode disposed on the substrate and an electrode disposed on the rod and the semiconductor rods being equally spaced from each other. This light-emitting element emits light in a direction either perpendicular to or in parallel with the surface of the substrate.
A semiconductor light-emitting element array including light-emitting elements and utilizing light emission from semiconductor rods to emit multiple colors of light is also known (see Patent Literature 2 and 3). For example, Patent Literature 2 proposes a light-emitting element including a multiple quantum well structure formed by repeatedly (e.g., three times) laminating a combination of a well layer and a barrier layer, to thereby emit multicolored light by adjusting the thickness of each well layer. It is also proposed that the emitted light having multiple colors is passed through a transparent film (or a wavelength-selecting filter) which transmits only a desired single color wavelength for wavelength selection of light.
Patent Literature 3 discloses a light-emitting element including two or more active layers, with each active layer having a pair of electrodes. This light-emitting element emits multicolored light upon application of a predetermined voltage to each electrode pair, thereby causing light emission from each active layer.
Selective-Area Metal Organic Vapor Phase Epitaxy (Selective-Area MOVPE) is known as one of important technologies to form nanopillar-shaped semiconductor rods (see Non-Patent Literature 1 and 2). MOVPE is a method to grow semiconductor crystals selectively at specific exposed areas of a semiconductor crystal substrate by MOVPE. It is reported in Non-Patent Literature 1 and 2 that the thickness and/or the height of the semiconductor crystals to be grown can be controlled by adjusting the exposed areas of the semiconductor crystal substrate. For example, the height of the grown rods (or nanopillars) becomes taller as the diameter of the exposed area is reduced, while the rods become taller when the spacing between the exposed areas (or pitch) is reduced. Needless to say, the rods become thicker as the area of the exposed portion is increased.
Meanwhile, technologies to fabricate a surface light-emitting element used for multiple wavelengths have been reported (see Non-Patent Literature 3 and 4). These documents relate to technologies for growing a thin film of a GaInAs/GaAs quantum well structure for the surface light-emitting element. In this technology, a plurality of linear and convex shaped steps (which are called “mesas”) are formed in parallel with each other on the surface of a GaAs (001) substrate. Subsequently, a multi-layered thin film is disposed on the substrate by MOCVD (Metal Organic Chemical Vapor Deposition). During the process, a thin film is disposed thickly on the top surface of the mesas, while a thinner film is disposed on the surface of the valley between the neighboring mesas.
The thickness of the thin film disposed on the surface of the valley between the mesas can be controlled depending on factors (which are also called “control parameters”), such as the width of the mesas, the depth (or height) of the steps, the spacing between the neighboring mesas, and the like. In other words, the distribution of the thickness of the film formed by MOCVD thin film epitaxy is determined by these control parameters and, therefore, the wavelength of light emitted from the light-emitting element fabricated by this method can be controlled. Proper control of the thickness of the thin film (or the wavelength of the light emitted from the light-emitting element) requires precise design and fabrication of the mesas.    Patent Literature 1: JP 4-212489 A    Patent Literature 2: JP 2003-347585 A    Patent Literature 3: JP 7-183576 A    Non-Patent Literature 1: Noborisaka, J. et al., “Catalyst-free growth of GaAs nanowires by selective-area metal organic vapor-phase epitaxy”, Applied Physics Letters, vol. 86, p. 213102-1-213102-3 (2005)    Non-Patent Literature 2: Yang, L. et al., “Size-dependent photoluminescence of hexagonal nanopillars with single InGaAs/GaAs quantum wells fabricated by selective-area metal organic vapor phase epitaxy”, Applied Physics Letters, vol. 89, pp. 203110-1-203110-3 (2006)    Non-Patent Literature 3: M. Arai, et al., “Multiple-wavelength GaInAs—GaAs vertical cavity surface emitting laser array with extended wavelength span”, IEEE Journal of Selected Topics in Quantum Electronics, vol. 9, No. 5, (2003) pp. 1367-1373    Non-Patent Literature 4: A. Onomura, et al., “Densely integrated multiple-wavelength vertical cavity surface-emitting laser array”, Japanese Journal of Applied Physics, vol., 42 (2003) pp. L529-L531.